This invention relates in general to the field of telecommunications and deals more particularly with a method and apparatus for applying messages and other communication signals to telephone lines.
Telecommunications networks include customer stations (typically ordinary telephone sets) which are linked together via switching centers in order to provide communication paths between the calling station and the called station. The switching center is usually a telephone company facility commonly known as a "central office", although it can also be a toll office, a private branch exchange (PBX) or a key telephone system. The switching function of the switching center is performed by a switching network which is controlled by a control complex and which at times switches various service circuits into the communication path. The service circuits include wrong number announcements and changed number announcements, digit receivers, tone generators and other peripheral equipment that performs specialized service functions. The transmission lines include customer lines which connect each customer station with a switching system and trunks which connect the switching systems with one another.
Telephone call setup involves, as the first step, the calling party taking his telephone "off hook", thus closing the switch hook contacts of the telephone to inform the local switching system of a request for service. The control complex of the switching system then applies a dial tone signal to the calling line and prepares to accept the digits that are to be dialed. When the number addressed by the calling customer station is dialed, the digits are accepted and interpreted by the control equipment at the central office. The initial digits (usually the first three) provide information as to whether the called station is serviced by the same or another central office. If the call is an inter-office call, the originating office selects an idle trunk to the terminating office if one is available.
When an available trunk is seized, it is connected with the calling customer's line through the switching network in the originating central office. The digits of the called number are then transmitted over the trunk to the terminating central office which determines the status of the called customer's line. In modern "common channel signaling systems", the called number is changed to a digital data message and sent over a separate circuit referred to as a "common channel signaling circuit". If the telephone at the called station is busy (off hook), an audible busy signal tone is applied to the calling party's line. If the called telephone is idle (on hook), a ring signal is applied to its customer line for activating the ringer in the called party's telephone. At the same time, a different but similar ringback signal is applied to the calling party's customer line by the ringback equipment in the terminating central office. In the newest "common channel signaling systems", a digital data signal to the originating central office may activate the application of the ringback signal to the calling party's line. The ringback signal is applied at 440 and 480 Hz, and takes the form of audible tones which are applied for a duration of two seconds each with an interval of 4 seconds between successive tones.
If the called telephone is answered, its off hook status causes termination of the ringback signal. At the end of the conversation, the telephones are returned to on hook status, and the customer lines and trunk revert to idle status. Intra-office calls are processed similarly, except that there is no need for seizure of a trunk because the call can be routed through the switching network in a single central office.
In recent years, telephone transmission facilities have been used to transmit various types of data in addition to ordinary telephone conversations. For example, it is now common for computers to communicate via telephone transmission lines. Facsimile machines and other modern machines transmit information over telephone lines. Stock market information, weather data, commodity prices, news and other data are also often transmitted by telephone. It is also common for music and/or commercial messages or announcements to be applied to telephone lines, especially when a party is placed on "hold" while waiting to speak to someone. However, to my knowledge, telephone lines have not been used to transmit messages or data during the ringback period when the calling party is waiting for the called party to answer.